ACIS Sub-Pixel Event Resolution

Analysis Guide

For sources near the optical axis of the telescope, the size of
the point spread function is smaller than the size of the ACIS
pixels. Applying the energy-dependent sub-pixel
event-repositioning (EDSER) algorithm can improve the image
quality of ACIS data for such sources.

About the PSF Calibration

The CXC believes that the sub-pixel event-resolution (SER)
algorithm does improve the ability to separate small scale
structure, but we have not yet calibrated ACIS on sub-pixel
scales, so obviously the potential for artifacts exists. As
reported last year to the Chandra Users' Committee
(Report,
PDF), we have a plan to enhance our simulation tools to
represent the SER-enhanced PSF. This is a significant
calibration, modelling, and development effort since we need to
model the grade distribution and have a much more detailed
internal model of the instrument. Refer to the
Chandra Ray Trace
caveats for some other limitations on the PSF
modelling tools.

In principle, sub-pixel analysis should not affect the HRMA ray
tracing. However, at the sub-arcsecond scale, HRMA calibration
is still on-going (e.g. Probing higher resolution:
an asymmetry in the Chandra PSF). Therefore, there may
be uncalibrated mirror-related effects at the subarcsecond scale
in addition to the unmodelled ACIS detector pixel effects.

1. Reprocessing the Data

The first step in sub-pixel data analysis is to ensure that the
data has been reprocessed with
pix_adj=EDSER. EDSER became the default parameter
value in CIAO 4.3, and has been the default in
standard data processing (SDP)
since version DS 8.4 (28 June 2011).

Before the use of EDSER, the coordinates of ACIS events were
randomized by +/- 0.5 pixel to avoid possible aliasing affects
associated with the CCD pixel grids. Information on that
correction is available in the pixel
randomization why topic.

The tool acis_process_events was modified in SDP version DS 10.4.2.1
(30 September 2015) and in CIAO 4.8 to make it possible to use the
subpixel algorithm for continuous-clocking mode datasets. Since the
subpixel adjustments affect not only the coordinates, but also the
times of such datasets, and since the algorithm can introduce
non-astrophysical features in the times of arrival, the adjustment is
not used in the standard data pipeline.

2. Run ChaRT to create a PSF

Dither and PSFs at Sub-Pixel-Scales

At present, ChaRT does not account for the spacecraft dither,
with information contained in the aspect solution file of an observation; thus limiting
the usefulness of ChaRT-generated PSFs at sub-pixel resolution
that are trying to match observations where the EDSER algorithm is
applied.

At sub-pixel resolutions, without the dither, the discrete set of
chip locations that result from applying the EDSER algorithm
translates into discrete sky positions, causing an aliasing effect
which is smoothed out by the aspect solution. SAOTrace v2 and
MARX v5 can account for the dithering.

The Chandra Ray Tracer
(ChaRT) simulates the best available point spread function
(PSF) for a point source at any off-axis
angle and for any energy or spectrum. Technical
details are available from the
About ChaRT page.

It is important to read
the caveats before
running ChaRT and using the resulting PSF simulations in
analysis.

The projected event file created with ChaRT and Marx contains X
and Y values without any pixel quantization, allowing a 2-D
histogram image can be made at any desired sub-pixel scale.

To create a sub-pixelated image with
dmcopy, specify a binning factor that is
less than 1. This example uses 0.1, and the limits have been
changed to +/- 25.6 pixels to create a 512 square pixel image: